This invention relates to a piston for internal combustion engines, preferably of an aluminum alloy, comprising machined, oil-dragging grooves formed on the periphery of the piston skirt and having concave transitions and a sawtoothlike profile with an obtuse profile angle.
During the power stroke of a piston of an internal combustion engine, a minute amount of combustion gas leaks at least past the upper compression rings. Because in the following cycle the compression rings again contact the hot top portion of the cylinder, in which the combustion has taken place, the lubrication of the upper compression rings is a complicated problem.
On the other hand, the rings on the piston should prevent an ingress of lubricant into the combustion chamber. However, the amount of lubricant which is available for the uppermost compression ring should be so large that the latter is sufficiently lubricated in spite of the high operating temperature. The lubication will be sufficient if the uppermost ring has neither burnt spots nor minute marks due to seizing and its wear is minimized, just as the wear of the cylinder and piston. A sufficient lubrication adjacent to the rings depends mainly on the formation and adherence of a hydrodynamic lubricant film between the piston skirt and the working surface of the cylinder even when there is only a scant supply of lubricant.
To enable the formation of a hydrodynamic lubricant film between the pitson skirt and the working surface of the cylinder, it has been usual for some years to provide particularly the piston skirt with a machined surface having a certain minimum roughness in a depth which is in most cases not less than 2-6 .mu.m, depending on the type of engine. Such a rough surface promotes the motion of the piston because the lubricant stored in the depressions of the surface forms a lubricating film and the rough surface also limits the wear between the piston skirt and the working surface of the cylinder.
To this end, it has been disclosed, e.g., in German Patent Application Sch No. 5976 Ia/46c.sup.1, to provide the piston skirt on its periphery with machined oil-retaining grooves, which have a semicircular or arcuate profile. As the piston is run in, the height of the profile decreases rapidly at first owing to the relatively high wear. Whereas this wear is limited as the contacting part of the profile increases, the susceptibility of the piston to seizing increases because the depth of roughness has decreased as a result of the wear so that only a relatively small amount of lubricant is retained in the oil-retaining grooves. As a result, the formation of a hydrodynamic lubricating film throughout the periphery of the piston skirt is not always ensured during the upward and downward movements of the piston.
In a two-stroke cycle engine, it is difficult to retain the relatively small amount of lubricant, which is obtained from the fuel mixture, between the piston skirt and the working surface of the cylinder and to raise said lubricant into the ring-carrying portion of the piston so that a hydrodynamic lubricant film can form on the periphery of the piston skirt.
German Patent No. 2,063,651 discloses oil-retaining grooves which are provided on the periphery of the piston skirt and have different side face angles and transition radii increasing toward infinity. The profile angle of the oil-retaining grooves is 100.degree.-170.degree., their smaller side face angle 0.5.degree.-10.degree. and their depth 1-20 .mu.m. During the operation of the internal combustion engine, a lubricant film of uniform thickness is formed between such piston and the working surface of the cylinder so that the hydrodynamic pressures of the lubricating film on the pressure and back-pressure sides are always balanced and ingress of excessive lubricant into the combustion chamber is precluded.
It has been found, however, that in internal combustion engines to be subjected to high thermal stresses, particularly in two-stroke cycle engines, the contacting surface area of the sawtooth-shaped profile of the above-mentioned iron layer is changed to such a large extent by the wear caused by the operation of the engine that the depth of the oil grooves is appreciably reduced so that lubricant in the amount required for the formation of a lubricant film of uniform thickness on the periphery of the piston is no longer stored in the oil-retaining grooves when the piston has been run in.